1. Field of the Invention
This invention relates to silicone rubber compositions for use in fluororesin-covered fixing rolls (hereinafter abbreviated in some cases as FFR silicone rubber compositions) and to fluororesin-covered fixing rolls. More particularly, this invention relates to an FFR silicone rubber composition that--even when cured at the relatively low temperatures necessary to avoid the development of wrinkles or creases in the surface of fluororesin-covered fixing rolls--is nevertheless capable of forming a silicone rubber with a low compression set and an excellent adherence to the roll shaft. The invention also relates to a highly reliable fluororesin-covered fixing roll.
2. Description of the Prior Art
Fluororesin-covered fixing rolls comprising a fluororesin layer formed on the circumference of a metal roll shaft across an intercalated layer of relatively low-durometer silicone rubber exhibit excellent toner release and make possible a reduction in equipment size and are therefore well-adapted for use in electrophotographic copiers, printers, facsimile machines, and the like (refer to Japanese Patent Application Laid Open (Kokai or Unexamined) Numbers Sho 53-74436 (1978), 57-89785 (1982), 59-74578 (1984), and 59-52269 (1984)).
Fluororesin-covered fixing rolls of this type are typically fabricated by preliminarily treating the circumference of the roll shaft with primer, inserting the treated roll shaft into a fluororesin tube, injecting a silicone rubber composition into the cavity created between the roll shaft and the tube, and then curing the silicone rubber composition. This process requires that the silicone rubber composition be cured at relatively high temperatures, for example, at 150.degree. C. to 200.degree. C., in order to achieve a satisfactory adhesion between the roll shaft and the silicone rubber layer. However, due to the difference in coefficient of thermal expansion between the silicone rubber composition and fluororesin, this heating episode frequently causes the appearance of creases or wrinkles in the surface of the resulting fixing roll. This problem is particularly noticeable with the use of silicone rubber compositions that yield low-durometer silicone rubbers, for example, with a JIS A durometer not exceeding 20.
When, on the other hand, the silicone rubber composition is cured at the relatively low temperatures, for example, 80.degree. C. to 120.degree. C., that avoid creasing in the surface of the fluororesin-covered fixing roll, the result is a poor adherence and intimacy of contact with the roll shaft and fluororesin that are in contact with the silicone rubber composition during the course of its cure. As a consequence, even the preliminary treatment of the roll shaft with primer does not yield much improvement in the adherence. Thus, while fluororesin-covered fixing rolls fabricated by a relatively low-temperature cure of the silicone rubber composition will not suffer from surface creasing, they will suffer from a poor reliability due to a ready tendency for delamination to occur at the roll shaft/silicone rubber layer interface.
As a result of extensive investigations directed to solving the problems described above, the present inventors discovered that an addition reaction-curing silicone rubber composition containing two different types of SiH-functional organopolysiloxane can be cured even at relatively low temperatures to give a silicone rubber with a low compression set and in particular an excellent adherence and intimacy of contact with a roll shaft and fluororesin in contact with the composition during its cure. With respect to these SiH-functional organopolysiloxanes, the inventors discovered in particular that the use of specific quantities of organopolysiloxane containing relatively large amounts of SiH can increase the adherence without significantly changing the physical properties of the silicone rubber product--most importantly without significantly changing its durometer. The present invention was achieved based on these discoveries.
In specific terms, then, the present invention takes as its object the introduction of an FFR silicone rubber composition that--even when cured at the relatively low temperatures necessary to avoid the development of creases in the surface of fluororesin-covered fixing rolls--can form a relatively low-durometer silicone rubber with a low compression set and an excellent adherence to the roll shaft. Another, corresponding object of the present invention is to provide highly reliable fluororesin-covered fixing rolls.
The silicone rubber composition according to the present invention is intended for the formation of the silicone rubber layer in fluororesin-covered fixing rolls in which a fluororesin layer is placed over the circumference of a roll shaft with a silicone rubber layer interposed between the fluororesin layer and the roll shaft.